The advantage of acrylic polymers as viscoelatic bases for pressure-sensitive adhesives are well known in the art. U.S. Pat. No. RE 24,906 (Ulrich) cites many examples of these products. Pressure-sensitive adhesives have been used for more than half a century for a variety of marking, holding, protecting, sealing, and masking purposes. The term "pressure-sensitive adhesive" (PSA) has a precise technical definition and has been discussed extensively in the technical literature, examples of which are Houwink and Salomon, Adhesion and Adhesives, Volume 2, Chapter 17, Elsivier Publishing Company, 1967, and Handbook of Pressure-Sensitive Technology (2nd Edition), Edited by Donatas Satas, Van Nostrand Reinhold Company, 1989.
Fundamentally, PSAs require a delicate balance of viscous and elastic properties which result in a 4-fold balance of adhesion, cohesion, stretchiness, and elasticity. In essence, PSA products have sufficient cohesiveness and elasticity so that, despite their tackiness, they can be handled with the fingers and removed from smooth surfaces without leaving residue.
There are several methods presently in use for the preparation of PSAs. These methods include batch, hot melt, solution, emulsion, suspension, ultra-violet (UV)-initiated bulk, and UV-initiated on-web polymerization techniques.
Acrylic polymer compositions may be made via solution polymerization. These solution polymerization methods require large amounts of organic solvents. Polymers in solvent may be difficult to handle and transport due to the volume of the solvent and the potential release of volatile organic compounds (VOCs) into the atmosphere. Using solvents also necessitates high heat or vacuum to remove the solvent from the polymers.
Methods of emulsion batch polymerization for pressure-sensitive adhesives have been disclosed in the art. Emulsions generally consist of thermodynamically unstable mixtures of two immiscible liquids. One liquid forms a dispersed phase of droplets in the other liquid. Emulsions may be oil-in-water (continuous water phase) or water-in-oil (continuous oil phase). In water-continuous polymerizable emulsions, the polymerizable materials and additives form small droplets (less than 1 micrometer in diameter) in a water medium.
Surfactants or emulsifiers are added to stabilize the emulsion. These stabilizing materials have a hydrophilic (water-attracting) portion, and a hydrophobic (water-repelling) portion. The stabilizing molecules form colloidal clusters called micelles. The hydrophobic portions of the molecules are drawn to the inside of the micelles, away from the water. The interiors of the micelles are the main polymerization sites for the hydrophobic oil-phase materials. The droplets of oil-phase materials are the source of monomers for the polymerization in the micelles. Any polymerization taking place in the oil droplets is referred to as suspension polymerization.
The emulsion polymerization process is started by water-soluble chemical free radical initiators, which are added to the emulsion and are activated by heat or UV radiation. An oil-soluble chemical free radical initiator may initiate suspension polymerization.
The polymerization reactions can be highly exothermic. To avoid an explosive heat generation and to achieve high molecular weight polymers, the rate of polymerization must be controlled. Polymerization takes place over several hours while the emulsion is agitated. The resulting polymerized materials may be used to form films and coatings, with the addition of a thickener or solvent.
Electron beams operate by bombarding molecules with electrons. These electrons displace other electrons in the bombarded molecules, thereby creating free radicals, which may react with other molecules. Electron-beam radiation produces a high rate of free-radical initiation and may produce free radicals in all components of the system including the product itself. Wilson, Radiation Chemistry of Monomers, Polymers, and Plastics, chapter 11, p. 375, New York, 1974. Because of this indiscriminate production of free radicals, electron-beam radiation is generally not used for polymerization processes, but instead is restricted to crosslinking processes.